Single ended signal is a commonly used signal form due to its simple processing circuitry. This signal however, is susceptible to interference during transferring process and consequently, where signal transmission system needs rigid requirement for anti-interference, differential signal transmission is normally employed. However, utilization of differential signal system increases complexity of hardware and results in limited application range. Thus, it is desired to provide a method of single ended signal transferring with interference-resistance.
Single ended signal may be easily influenced by interference during transmission. Due to lack of knowledge about reasons of occurrence of interference, it is high costly for normal method of reducing interference. Method for decreasing interference currently used is reducing interference source, as well as impedance of the connections at reference terminal as soon as possible. Thus, it is desired to provide an improved method of signal transferring capable of resisting high interference.
The follow is analysis of generation of interference.
FIG. 1 is an equivalent circuit of a single ended signal transferring circuit, wherein
G: reference ground
Goe: signal output device ground
Gie: signal input device ground
Vs: voltage source of output signal
Rs: internal resistance of output signal
Ri: internal resistance of signal input side
Cs: the sum of body capacitance and capacitance of induced capacitor of the signal output device ground relative to the reference ground G
Vns: induced interference voltage of the signal output device ground relative to the reference ground G
Ci: the sum of body capacitance and capacitance of induced capacitor of the signal input device ground relative to the reference ground G
Vni: induced interference voltage of the signal input device ground relative to the reference ground G
Rr: impedance of wiring between the reference terminal of signal input and output side
Vnl: induced interference voltage on the wiring between the reference terminal of signal input and output side
Vnr: interference voltage resulted from interference current generated at Rr by all the interference source existing in a system
Vn: the sum of Vnl and Vnr, i.e., equivalent interference voltage at the signal input side
The single ended signal has a signal terminal and a reference terminal, the later being commonly referred as a signal ground. The reference terminal has its potential constant, whereas potential at the signal terminal varies. It can be considered that interference mostly coming from wiring of the reference terminal and interference coming from the signal terminal is very little during transmission of the single ended signal. The interference voltage results from the following three sources: the first source is induced interference voltage at wiring between the reference terminal of receiving side and transmitting side, whose equivalent voltage to input signal is represented by Vnl in the figure; the second source is potential difference at the impedance Rr of wiring of the reference terminal, the potential difference resulting from interference current generated by unbalanced inducing voltage between the reference terminal of receiving and transmitting sides, the equivalent interference voltage to the input signal is shown in the figure by Vnr, and in addition, Cs, Vns, Ci and Vni are equivalent circuits of the interference source; and finally, the third source is multi-ground wire loop interference which may occur depending upon connection of the ground wires. Now, detailed analysis of influence of these interferences is provided.
Induced interference at the wiring between the reference terminal of receiving and transmitting sides is explained as follows.
Vnl shown in FIG. 1 is the interference voltage induced by this interference source. Single ended signal is generally transferred by unbalanced shielded wire. As the reference terminal of the signal is connected by an outer shielded layer of the shielded wire, it will easily generate induced voltage. The interference source may be equivalent to a voltage source with a high internal resistance. This interference voltage can be reduced by reducing impedance of load between its two terminals. The load impedance is impedance between Goe and Gie with the connection wiring of the reference terminal removed, and consisted of two impedances in parallel. One impedance is combination of Rs and Ri in series and the other is combination of Cs and Ci in series. Vnl in the figure is the interference voltage with the load impedance between the two terminals being calculated. In the equivalent circuit shown in FIG. 1, since the signal reference terminal and device ground are the same, and since Cs and Ci are large enough, Vnl is not large and therefore, Vnl has less impact on entire input interference voltage. It should be noted that thickening of the conductor will reduce the internal resistance of the interference source thereof and increase the induced interference signal, hence resulting in increase of Vnl.
The receiving and transmitting terminal employ shielded inner conductor as their connection wiring and accordingly, less interference signal will be induced on the wire. However, in case where the load impedance become large due to large input impedance of the receiving terminal, or where the shielded wire produces less effective shielding function, induced interference influence from connection wiring between the receiving and transmitting terminals will increase.
If the single ended signal is transferred via balanced shielded wire, that is, both the signal terminal and reference terminal use shielded wire incorporating inner conductor therein, the induced interference potential at the connection wiring can be regarded as very small and consequently, less influence will be applied to Vnl.
Next, impedance of the connection wiring at the reference terminal resulted from interference current passing through the connection wiring is explained, wherein the interference current is induced by unbalancing induced voltage between the reference terminal of receiving and transmitting sides.
Cs, Vns, Ci, and Vni shown in FIG. 1 are the equivalent circuit of the interference source. The interference source includes induced interference and interference resulted from power and grounding system. Cs is the totality of body capacitance and induced capacitance of the device ground of transmitting side, Vns is induced voltage interference on the device ground of the transmitting side, Ci is the totality of body capacitance and induced capacitance of the device ground of receiving end, while Vni is induced voltage interference on the device ground of the receiving side. When induced potential difference occurs between Vns and Vni, induced current will be generated, and accordingly, interference voltage Vnr is generated on Rr.
In FIG. 1, as the receiving and transmitting signal reference terminals directly couple with the device grounds, both Cs and Ci are very large and therefore, induced interference voltage will also be very large. In case where the device ground is still coupled to an external ground system, Cs or Ci thereof will still increase. Consequently, interference current flowing through Rr will also be increased if induced potential difference exists between the reference terminals of receiving and the transmitting sides, thus resulting in larger interference voltage.
Multi-ground wire loop interference is explained as follows.
During transmission of single ended signal, there may also be multi-ground wire loop interference. If the devices at both sides are connected by more than one ground wires, a loop circuit will be defined between every two ground wires. Interference potential will be produced in the loop, once varying flux passes through the area defined by the loop. The interference current resulted from the interference potential will generate interference voltage on signal ground wire Rr. In actual applications, this situation may occur frequently. For example, when both the device themselves have its power and are connected with the signal ground wires, one power will be connected with the ground wire in case where the two device are powered by the same power. Further, since another signal ground wire is needed for transmission of the single ended signal, there will accordingly be two ground wires. However, in conventional single ended signal transmission system, the device ground and signal ground are usually connected with each other directly. Once ground wire loop interference occurs, much experience will be required to eliminate the interference and thus, handling of the ground wire will become complicated.
In scheme of the invention, as isolation impedance is disposed in the loop composed of the signal ground and device ground, and since the impedance is sufficiently larger than impedance of the signal ground, ground wire loop interference voltage will mainly apply on the isolation impedance and will have little impact on the signal ground.